The invention relates to a method for determining the weight of a probe of a coordinate measuring machine wherein the probe is connected to a probe head.
U.S. Pat. No. 5,966,681 discloses a method for controlling coordinate measuring apparatus while taking into consideration the weight of the probe. The path acceleration of the probe head is ascertained to determine the weight of the probe. The acceleration of the probe head is determined from the derivation of the time-dependent change of the measured probe head positions or from the derivation of the desired values.
U.S. Pat. No. 5,966,681 discloses a further dynamic method for determining the weight of the probe. In this method, the probe is guided along a circular path. The probe is held in a rest position which the probe assumes in the rest state, by the application of force to the probe. This force is required to hold the probe in the rest position and the weight of the probe is determined from this force.
As already described in U.S. Pat. No. 5,966,681, the determination of the weight of the probe is preferably undertaken in the built-in state because then the determined weight is already present in the system and therefore input errors at the user end are prevented. On the other hand, the weight of the probe is determined already in the configuration provided for the operation. Dynamic methods, however, have the disadvantage that the determined values are subjected to large measurement errors.
Usually, coordinate measuring machines are operable with different probes which are selected in dependence upon the object to be measured. Often, the operators of the coordinate measuring machines themselves configure the probes adapted to their requirements. It is, in principle, possible to already consider the weight of probes in the control of the coordinate measuring machine by the manufacturer of coordinate measuring machines and/or coordinate measuring apparatus. However, this would mean a substantial limitation for the operator who then could utilize only probes offered by the manufacturer of the coordinate measuring machine in limited numbers. Such a significant limitation is, however, in no way desirable.
For the operators of coordinate measuring apparatus, it is desirable to have the freedom to be able to utilize their own probes. However, to be able to continue to guarantee a high measurement accuracy of the coordinate measuring machines (especially at high driving speeds over the surface to be measured), it is necessary that the weight of the probe used at a particular time be known as precisely as possible in order to be able to compensate for occurring centrifugal forces in dependence upon the scanning speed.
It is an object of the invention to provide an additional and simplified method for determining the weight of the probe which is connected to the probe head. It is a further object of the invention to increase the accuracy in the determination of the weight of the probe.
The method of the invention is for determining the weight of a probe of a coordinate measuring machine having a control unit and a probe assembly which includes a probe head and the probe connected to the probe head. The method includes the steps of: determining the weight of the probe without an active control of the movement of the probe by the coordinate measuring machine; and, conducting signals from the probe assembly to the control unit.
The weight of the probe is determined by means of signals supplied to the control unit from the probe and/or probe head. With this measure, the measuring accuracy can be increased in that the control unit is supplied with signals from the probe and/or probe head. During the determination of the weight, the probe is not driven by the coordinate measuring machine to carry out a movement.
In order to guarantee the determination of the weight of the probe with high accuracy, it has been shown to be advantageous to determine the weight of the probe statically or with the probe freely oscillating. For a static determination, the probe remains in a position during the determination and, for a freely dynamic determination, the probe returns to its rest position from a deflected position without a force acting externally on the probe, that is, from the probe head.
Values averaged over time can be applied in the static determination of the weight of the probe whereby the measuring accuracy is increased. In a static determination, no damping effects occur which are based on the configuration of the probe head and which can be temperature dependent.
Via the simple allocation of a force, which is introduced via the probe head, and an assumed position, which deviates from the rest position (also characterized as a deflection), a conclusion as to the weight of the probe can be drawn from the ratio of the force to the distance. In individual preferred embodiments, it is provided that the probe is charged with a predetermined force whereby a direct conclusion can be drawn as to the weight of the probe from the assumed deflection position while considering a calibration curve.
In a further embodiment, it is provided that a predetermined deflection position is driven to with the probe. The weight of the probe can be determined from the required force while considering a corresponding calibration curve.
It has been shown to be advantageous to provide the probe with an element converting an applied force into electrical signals. Especially piezo elements have been shown to be suitable and are cost effective and operate reliably. These piezo elements exhibit a high internal stiffness whereby an unwanted falsification of the measurement values with respect to the Z component does not occur.
It has been shown to be advantageous to connect the probe to the probe head while interposing the converting element so that at the conclusion of the assembly of the probe, the signals, which characterize its weight, can already be conducted to the control unit. The converting element is arranged between the probe head and a probe head weighing element. This converting element is assigned to a weighing cell which is fixedly connected to the bounding components, for example, by means of adhesive, threaded fasteners, or the like.
If the deflection out of the zero position for the determination of the weight of the probe is stored in the control unit with a corresponding calibration curve of signal value to weight of the probe, then a conclusion can be drawn as to the weight of the probe in dependence upon the supplied signals. The zero position is the position which is assumed by the probe head without the probe.
A probe head can be provided with a compensating spring to compensate for the inherent weight of the probe. For such a probe head, it has been shown to be advantageous that the extent of the displacement of the compensation spring can be applied for the determination of the weight of the probe. Such compensation springs have a first suspension point which can be vertically displaced, for example, by driving a corresponding electric motor. A second suspension point is operatively connected to the probe. Especially electric motors can be provided which are available as standard components and which are provided with incremental transducers so that the weight of the probe is determined by the increments which are detected when driving the electric motor to compensate for the weight of the probe.
If the electric motor has no sensors for recording the adjusted distance, then, as an alternative, it can be provided that the weight of a probe is determined in that the deflection in the z direction is recorded in that the deflection in the z direction is compensated for a short time by the application of current to an actuating device which is provided. In this way, a current is detected which characterizes the deflection. The above deflection in the z direction is based on the weight of the probe. A conclusion as to the weight of the probe can be drawn from the current when a calibration curve is provided.
If the spring constant of the probe head with respect to the z direction (that is, the work direction of gravity) is known, then after taking up the probe, a conclusion can be drawn as to the weight of the probe directly via the recording of the deflection of the probe head.
For a free dynamic determination of the probe weight, the probe is first deflected out of the rest position or zero position. The probe returns to the rest position after being released from this deflection position, that is, it swings back without any action by the coordinate measuring machine. The signals, which characterize the natural frequency of the free swinging system, are supplied to the control unit. The weight of the probe can be derived from the determined natural frequency. In this way, an especially simple method for determining the weight of the probe is made available. In this method, it is not necessary that the assumed deflection position is known nor is the knowledge of the force necessary for the deflection required. The deflection must only be sufficiently large so that the period of oscillation of the free swinging system can be determined with the required accuracy, if needed, by averaging over several detected oscillation periods.
In this system, it has been shown to be advantageous to draw a conclusion as to the weight of the probe in that the weight of the probe is recorded with reference to the natural frequency of the system without the probe and from the change of the natural frequency with the probe. From the change of the natural frequency, a determination of the weight of the probe is made by means of a calibration curve which is preferably stored in a data memory assigned to the control unit. Especially the computation complexity, which is required for the determination of the weight, can be reduced with the stored calibration curves.
In order to determine the weight of the probe with the greatest possible accuracy, it is necessary that the period of oscillation T for the natural frequency is changed as greatly as possible in dependence upon the weight of the probe. The quantity C=Coxc2x110% has been shown to be a suitable spring constant wherein: Co=2go/L with L=the length of the freely oscillating system and go=the weight of the oscillating system without probe.
In a preferred embodiment, it is provided to select a spring constant for the system. For the selection of the spring constant, the estimated weight gT of the probe is considered. Accordingly, g=go+gT applies wherein:       C    o    =            2      ⁢              (                              g            o                    +                      g            T                          )              L  
and
C=Coxc2x110%.
It has been advantageously shown to determine the spring constant of the probe head in that the probe head is pivoted by 90xc2x0 into the horizontal position. Then, the probe head is deflected in the horizontal plane and the spring constant results from the ratio of deflection to required force.
A further advantage, independent of the selected method for determining the weight, is that, when the weight of the probe used is known, an overloading especially of the spring system in the probe head is prevented in that, for a weight of the probe, which exceeds a maximum value, the measuring operation is not permitted with the coordinate measuring machine. In this way, the operator is simultaneously signaled that the mounted probe is not suitable for the operation with this coordinate measuring machine.